Field of the Invention
This disclosure relates generally to the field of instrumentation in freestream flow fields. In particular, features for improving the flow over an optical instrument in a freestream flow field are disclosed.
Description of the Related Art
Aberrations in electromagnetic energy induced by unfavorable fluid flow are a serious concern in systems that operate in flow fields. For instance, in some airborne communication and imaging systems, an optical beam is required to be transmitted through a relatively long distance, over which the quality of the beam can degrade due to variations of the index of refraction along its path. For air and many fluids, the refractive index is linearly related to the density of the fluid through the Gladstone-Dale relation, and therefore density fluctuations, such as in turbulent flow, are the root cause of these aberrations.
An electromagnetic beam in a flow field generally encounters flow in the vicinity of the instrument that is detrimental to successful operation of the instrument. For instance, turbulent flow in the vicinity of an aperture of an instrument may be produced by the presence of solid boundaries. These near-field flows typically involve turbulent boundary layers, mixing layers, and wakes. When an initially planar electromagnetic wavefront, such as an optical wavefront, passes through a flow field, different parts of the wavefront experience different densities in the medium and hence have different propagation speeds. Consequently the wavefront becomes deformed. A small initial deformation of the wavefront can lead to large errors on a distant target. The consequences of such deformations include beam deflection (bore-sight error) and jitter, beam spread, and loss of intensity. Wavefront distortions can also cause reductions of resolution, contrast, effective range, and/or sensitivity for airborne electro-optical sensors and imaging systems.
Conventional solutions addressing the problems due to such unfavorable near-field flow have included openings or channels through the vehicle or structure housing an instrument. Other approaches have involved actively influencing the flow with jets of fluid injected into the flow. Still other approaches have involved flow detection and responsive beam correction. However, these approaches are invasive, require complex sensors and systems, and/or necessitate alterations to the structure of the vehicle containing the housing. Further, these approaches limit the utility of the instruments. For instance, the orientations of the instrument with respect to the freestream flow, for which the instrument will successfully operate, are limited. As an example, approaches that employ active control with static jets of fluid limit the viewing angles of the instrument because the flow from the jets is at a different angle relative to the flow over the instrument after the instrument has rotated. While the jets themselves may move to compensate for the moving instrument, this introduces even further complexity and cost.
It is therefore desirable to address the problems associated with near-field flow over an electromagnetic instrument in a more convenient, economical, and less complex manner.